The longterm goal of our laboratory has been the investigation of structure and organization of genomes and its effect on accumulation of base substitutions. Based on nucleotide sequence information and the banding profiles of nuclear DNA in equilibrium density gradients, the genomes of warm-blooded vertebrates appear to be partitioned into large tracts of relatively homogeneous base composition. This structuring implies that the rates and patterns of molecular evolution, even at sites considered to be effectively neutral, are not uniform over the genome. We have developed a procedure which allows the rapid determination of the nucleotide content on very small quantities of DNA. By applying this technique to clones encompassing extensive segments of the human genome, we plan to investigate: (1) the dimension and distribution of these large regions of uniform base composition within the genome, (2) the correspondence between the compartmentalization of the genome and the Geimsa-stained bands of human chromosomes and (3) the substitution rates of genes positioned within regions of distinct base compositions. In addition, we will compare the genome organization detected in humans to that in a non-mammalian species known to have both striking chromosome banding-patterns and variation in evolutionary rates. Cumulatively, the proposed studies will directly evaluate the magnitude of structuring in eukaryotic genomes and examine the consequences of genome organization on the evolution of DNA sequences.